Stretching out for a snooze after a Sunday roast is hard to resist - and now scientists have explained why.

University of Manchester researchers have discovered how the nerve cells in the brain that keep us alert become turned off after we eat.

Glucose - the sugar found in foods - stops these cells from producing signals to keep people awake.

The study, in Neuron, could help treat obesity and eating disorders and aid understanding levels of consciousness.

We naturally have a dip in alertness around 2pm to 4pm that happens whether we eat lunch or not

Neil Stanley, sleep expert

The human body has an in-built mechanism which means that when the body needs fuel, the brain chemistry creates alertness.

But when that hunger is sated, the chemistry swings the other way.

The University of Manchester team looked at how glucose regulates how alert and energetic someone feels.

They focused on nerve cells in the brain that produce tiny proteins called orexins.

It was known that these cells, which promote wakefulness and which can lead to sleep disorders and obesity if they are faulty, were sensitive to glucose levels.

But it was not known how they were affected.

'Hunger keeps you awake'

In this study, researchers genetically engineered mice to produce a fluorescent protein in the orexin neurons so that post mortem lab tests could show how they behaved in response to small changes in glucose levels which occur in normal daily cycles of hunger and eating.

It was found that glucose inhibits the behaviour of orexin neurons by acting on a type of potassium ion channels.

These are pore-like proteins in a cell membranes which affect how a cell behaves by controlling the flow of potassium, which provides energy to cells.

Glucose appears to inhibit this flow, and therefore the signalling ability of the cells.

Dr Denis Burdakov, who led the research, said: "We have identified the pore in the membrane of orexin-producing cells that is responsible for the inhibiting effect of glucose.

"This previously unknown mechanism is so sensitive it can detect minute changes in glucose levels - the type that occurs between meals for example.

"This may well provide an explanation for after-meal tiredness and why it is difficult to sleep when hungry."

Breakfast 'no tiredness link'

He added: "Now we know how glucose stops orexin neurons 'firing', we have a better understanding of what may occur in disorders of sleep and body weight.

"This research perhaps sheds light on why our European friends are so fond of their siestas."

Neil Stanley, director of sleep research at the Human Psychopharmacology Research Unit Medical Research Centre at the University of Surrey, said the work was interesting.

But he added: "There are many behavioural, circadian and societal factors that also determine whether we nap in the afternoon or not.

"The siesta is not only dependent on the intake of food.

"We naturally have a dip in alertness around 2pm to 4pm that happens whether we eat lunch or not.

"We also do not get tired after eating breakfast because we are on the rising phase of our circadian rhythm."